TDD (Time Division Duplex) systems transmit downstream (network to subscriber) data and upstream (subscriber to network) data in distinct time slots of the same physical channel. Additionally, there is often a small guard-time between distinct time slots to ensure that data does not overlap. One new TDD system is called ‘G.fast,’ which is currently in the process of standardization by the ITU-T (International Telecommunication Union—Telecommunication Standardization Sector). G.fast is to transmit over relatively short (<250 m) copper telephone loops and premises wiring.
FIG. 1 is a diagram of slots on a horizontal time axis for a classic TDD system, alternately transmitting downstream from the network toward the subscriber and upstream from the subscriber toward the network in different Down and Up TDD time slots. There is a downstream slot 12, and an upstream slot 14. This is followed by another downstream slot 16 and then an upstream slot 18. The cycle is repeated along the horizontal time axis. The “asymmetry ratio” is the ratio of the size of the Down slot compared to the size of the Up slot. There is usually also a small guard-time (not shown) between each time slot. Each repetition of the two slots may be referred to as a frame. Alternatively, a frame may have multiple repetitions of the two slots.
To reduce power consumption and interference on nearby lines, a power saving TDD system is shown in FIG. 2. FIG. 2 is a diagram of slots on a horizontal axis for the power saving TDD system. Broadband data communication systems are often idle or highly underutilized, and current practice is generally to send idle codes at full power when there is no data traffic. A power saving alternative is to suppress transmission when there is no user data traffic. As shown in FIG. 2, there are two frames 21, 22, TDD frame1, TDD frame2, each having a downstream slot, 23, 27, followed by an upstream slot, 25, 29. In addition, a portion of the downstream slot, in this case the later portion, is an off portion, D_off 24, 28, during which transmission is suppressed. During the off portion, neither data nor idle bits are sent. Similarly the upstream slot, Up, has an off portion U_off 26, 30 at the end of each slot, during which neither data, nor idle bits are sent. Sending no power, during off times “D_off” and “U_off” can provide significant power savings. “D_off” and “U_off” may also be combined into a single off portion.
In many TDD systems, there are multiple physical channels. If these channels are close together in location and overlap in frequency, then they may interfere with each other. FIG. 3 is a diagram of a TDD system with two channels 31, 32. In this example, each channel uses a twisted pair of copper cables and both channels are in the same cable or cable binder. As illustrated in FIG. 3, systems transmitting in multi-pair copper cables can generate crosstalk into each other, Near-End Crosstalk (NEXT) 33 and Far-End Crosstalk (FEXT) 34 are generated by each channel and directed at least in part into the nearby channel.
Each channel is connected between a network-end Transmission Unit-Office (TU-O) 35, 36 and a subscriber-end Transmission Unit-Remote (TU-R) transceiver 37, 38. While each channel is shown as connecting a different TU-O to its own TU-R, a single TU-O can connect to a single TU-R using both of the two channels. This allows more data to be sent to the one TU-R. A single TU-O may also connect to multiple TU-Rs, which is sometimes called “bonding.”
NEXT can be very powerful and debilitate high speed transmissions. ADSL (Asymmetric Digital Subscriber Line) and VDSL (Very high bit rate Digital Subscriber Line) use Frequency-Division Multiplexing (FDM) to avoid self-NEXT. Self-NEXT 33 is the crosstalk that is created into neighboring channels as shown in FIG. 3.